FIG. 11 is a perspective view illustrating a short-wavelength semiconductor laser having an oscillation wavelength of 0.98 .mu.m. In the figure, reference numeral 1 designates an n type GaAs substrate having a (001) surface orientation. There are successively disposed on the (001) surface of the GaAs substrate 1, an n type GaAs buffer layer 2 having a carrier concentration of 1.times.10.sup.18 cm.sup.-3, a 1.5 .mu.m thick n type AlGaAs lower cladding layer 3 having a carrier concentration of 1.times.10.sup.18 cm.sup.-3, an undoped multiquantum well (MQW) active layer 4 comprising InGaAs layers and GaAs layers alternatingly arranged, a 1.5 .mu.m thick p type AlGaAs upper cladding layer 16 having a carrier concentration of 1.times.10.sup..about. cm.sup.-3, and a 0.5 .mu.m thick p type GaAs contact layer 8 having a carrier concentration of 1.times.10.sup.19 cm.sup.-3. Two grooves, each about 20 .mu.m wide, penetrate through the contact layer 8 and reach into the upper cladding layer 16, extending along the cavity length direction of the laser. A region sandwiched between these grooves is a ridge structure 10. The ridge structure 10 is about 1 .mu.m wide at the top. An insulating film 11, such as SiO.sub.2, is disposed on the contact layer 8 and on the inner surfaces of the grooves. This insulating film 11 has an aperture 11a about 1 .mu.m wide on the upper flat surface of the ridge structure 10. A p side electrode 12 is disposed over the insulating film 11, covering the aperture 11a. That is, the p side electrode 12 is in ohmic contact with the contact layer 8 through the aperture 11a. An n side electrode 13 is disposed on the rear surface of the substrate 1. Further, the laser has a pair of cavity facets 14 perpendicular to the longitudinal direction of the ridge structure 10.
FIGS. 12(a)-12(d) are cross-sectional views illustrating process steps in a method of fabricating the semiconductor laser shown in FIG. 11. In the figures, the same reference numerals as those shown in FIG. 11 designate the same or corresponding parts. Hereinafter, the fabrication method will be described using FIGS. 12(a)-12(d).
Initially, as shown in FIG. 12(a), the n type GaAs buffer layer 2, the n type AlGaAs cladding layer 3, the MQW active layer 4, the p type AlGaAs upper cladding layer 16, and the p type GaAs contact layer 8 are successively grown on the n type GaAs substrate 1 in the wafer state by MOCVD (Metal Organic Chemical Vapor Deposition).
In the step of FIG. 12(b), an SiO.sub.2 film is deposited over the entire surface of the contact layer 8 by sputtering, and the SiO.sub.2 film is patterned by an exposure technique and an etching technique, for example, wet etching using a hydrofluoric acid based etchant or dry etching using CF.sub.4 gas, thereby forming an SiO.sub.2 mask pattern 15 having parallel two grooves 15a extending in what becomes the cavity length direction of the laser. The width of the center portion of the mask pattern 15 sandwiched between the two grooves 15a is about 1 .mu.m, and the width of the groove 15a is about 20 .mu.m.
In the step of FIG. 12(c), using the mask pattern 15, the p type GaAs contact layer 8 and an upper portion of the p type AlGaAs upper cladding layer 16 are wet-etched using a tartaric acid based etchant. The etching depth is 1.1 .mu.m. As a result of the etching, the ridge structure 10 is formed under the center portion of the mask pattern 15.
After removal of the mask pattern 15, as shown in FIG. 12(d), an insulating film, such as an SiO.sub.2 film, is again deposited over the entire surface of the substrate by sputtering, and an aperture about 1 .mu.m wide is formed in the insulating film on the upper flat surface of the ridge structure 10 using a photolithographic technique, thereby forming an insulating film 11 having an aperture 11a.
Thereafter, the p side electrode 12 is formed on the insulating film 11 and on the contact layer 8 exposed in the aperture 11a, and the n side electrode 13 is formed on the rear surface of the substrate 1. Finally, the cavity facets 14 are formed by cleaving, and the wafer is divided into individual elements, completing a semiconductor laser as shown in FIG. 11.
A description is given of the operating principle of the semiconductor laser. Holes supplied from the p side electrode are concentrated in the ridge structure 10, which has a trapezoidal cross section perpendicular to the cavity length direction, and efficiently injected into the active layer 4. Meanwhile, electrons are injected into the active layer 4 from the n side electrode 13. In the active layer 4, holes and electrons recombine to produce light. In the direction perpendicular to the surface of the substrate 1 (hereinafter referred to as "vertical direction"), the light is confined in the active layer 4 by the upper and lower cladding layers 3 and 16 disposed on and beneath the active layer 4. In the direction parallel to the surface of the substrate 1 and perpendicular to the cavity length direction (hereinafter referred to as "horizontal direction"), the light is confined in the ridge structure 10 due to the distribution of effective refractive indices in the horizontal direction because there exists air, of which refractive index is smaller than that of a semiconductor layer, on both sides of the ridge structure 10.
The far field radiation angle of the laser light emitted from the cavity facet 14 of the semiconductor laser in the direction perpendicular to the surface of the substrate 1 is 27.degree., and the radiation angle in the direction parallel to the surface of the substrate 1 is 8.degree.. That is, the far field is an oval that is longer in the vertical direction. When this laser light is coupled with an optical fiber, the coupling efficiency is 20.about.30% degraded as compared with a case where circular laser light is coupled with an optical fiber. Therefore, in order to obtain a sufficient coupling efficiency between laser light and an optical fiber using the conventional semiconductor laser, the output power of laser light must be increased. However, high-power output operation very likely deteriorates the semiconductor laser, resulting in a reduction in reliability.
Further, in the semiconductor laser, since laser light is confined in and guided through the active layer 4, the light emitting spot at the cavity facet 14 is small, so that the laser light widely spreads when emitted from the cavity facet. Since the spot size of the emitted laser light is large, a lens for coupling the laser light with an optical fiber must be disposed between the semiconductor laser and the optical fiber, resulting in an increase in assembly cost because process steps of positioning and fixing the lens are required.
Further, when the conventional semiconductor laser oscillates at a high power, about 200 mW, COD (Catastrophic Optical Damage) may occur because of light absorption at the cavity facet. This means that the reliability of the laser degrades with the passage of time.
The above-mentioned problems are solved in a spot size changeable semiconductor laser in which a gain part generating light and a spot size changing part are integrated, which laser is disclosed in, for example, Electronics Letters, Vol.31 (1995), p.1071.
FIG. 21 shows a perspective view of a semiconductor laser disclosed in Japanese Published Patent Application (examined) No. Hei.8-31652. This semiconductor laser is fabricated as follows. A pair of stripe-shaped SiO.sub.2 films 69 are formed on a prescribed region of a p type GaAs substrate 61 so that a stripe-shaped portion of the substrate 61 is exposed between the films 69 and, using the SiO.sub.2 films 69 as masks for selective growth, a p type Al.sub.x Ga.sub.1-x As cladding layer 62, a p type Al.sub.y Ga.sub.1-y As active layer 63, an n type Al.sub.x Ga.sub.1-x As cladding layer 64, and an n type GaAs contact layer 65 are grown on the substrate 61. In the semiconductor laser so fabricated, since the active layer 63 is thinner in a region adjacent the cavity facet of the laser than in another region, the amount of light leaking from the active layer 63 into the upper and lower cladding layers increases in the vicinity of the facet, so that the light emitting spot at the facet increases and the width of a radial beam emitted from the facet narrows. In addition, since the light density at the laser facet is reduced, COD hardly occurs, whereby high-power output operation is achieved.
FIG. 13(a) is a perspective view illustrating a conventional spot-size changeable semiconductor laser, and FIG. 13(b) is a cross-sectional view taken along a line 13b--13b in FIG. 13(a). This semiconductor laser is a long-wavelength semiconductor laser in which a spot size changing part 200 and a gain part 300 for generating laser light are integrated on an n type InP substrate 101. There are successively disposed on the n type InP substrate 101, an n type InP lower cladding layer 102, an undoped MQW active layer 103 comprising, alternatingly arranged, two kinds of InGaAsP layers of different compositions, and a p type InP first upper cladding layer 104. The first upper cladding layer 104, the active layer 103, and the lower cladding layer 102 form a stripe-shaped ridge structure 113 having a prescribed width. P type InP current blocking layers 105 and n type InP current blocking layers 106 are disposed on both sides of the ridge structure 113 to bury the ridge structure 113. A p type InP second upper cladding layer 107 is disposed on the ridge structure 113 and on the n type InP current blocking layer 106, and a p type InGaAs contact layer 108 is disposed on the second upper cladding layer 107. The lower cladding layer 102, the active layer 103, and the first upper cladding layer 104 have uniform thicknesses in the gain part 300. In the spot size changing part 200, the thicknesses of these layers gradually decrease with distance from the gain part 300. A p side electrode 109 is disposed on the contact layer 108 in the gain part 300. An n side electrode 110 is disposed on the rear surface of the substrate 101.
FIGS. 14(a)-14(d) are perspective views illustrating process steps in a method of fabricating the spot-size changeable semiconductor laser shown in FIGS. 13(a) and 13(b). In these figures, reference numeral 300a designates a gain region on the substrate 101 where the gain part 300 will be produced, and reference numeral 200a designates a spot size changing region on the substrate 101 where the spot size changing part 200 will be produced.
Initially, as shown in FIG. 14(a), an insulating film comprising SiO.sub.2 or the like is deposited over the n type InP substrate 101, and prescribed portions of the insulating film are removed using exposure and etching techniques, thereby forming a mask pattern 111 for selective growth. The mask pattern 111 comprises a pair of insulating films having stripe-shaped (rectangle) surfaces and extending in the direction along which the gain region 300a and the spot size changing region 200a are arranged, i.e., in what becomes the cavity length direction of the laser. Further, these insulating films are disposed parallel to each other with a prescribed spacing between them.
In the step of FIG. 14(b), using the insulating films 111 as masks, the lower cladding layer 102, the MQW active layer 103, and the first upper cladding layer 104 are successively grown on the substrate 101 by MOCVD. During the MOCVD growth, since the masks 111 are present on the substrate 101, the growth rate of the crystalline semiconductor varies on the surface of the substrate 101. To be specific, the growth rate is higher on the region between the masks 111 in the gain region 300a than on the spot size changing region 200a. Further, on the spot size changing region 200a, the growth rate gradually decreases with distance from the masks 111. As a result, the respective semiconductor layers grown on the substrate 101 have uniform thicknesses in the gain region 300a and, in the spot size changing region 200a, the thicknesses of the semiconductor layers gradually decrease, relative to those in the gain region 300a, with distance from the gain region 300a.
In the step of FIG. 14(c), after removal of the masks 111, a mask 112 is formed on the first upper cladding layer 104. The mask 112 is a stripe having a prescribed width and extending in what becomes the cavity length direction of the laser. Using the mask 112 as a mask for selective etching, wet etching is carried out from the surface of the first upper cladding layer 104 until reaching the substrate 101, thereby forming the ridge structure 113 under the mask 112.
In the step of FIG. 14(d)), using the mask 112 as a mask for selective growth, the p type InP current blocking layers 105 and the n type InP current blocking layers 106 are successively grown on the substrate 101 so that the ridge structure 113 is buried in the current blocking layers 105 and 106.
After removal of the mask 112, the second upper cladding layer 107 and the contact layer 108 are successively grown on the ridge structure 113 and on the n type InP current blocking layers 106. Thereafter, the p side electrode 109 is formed on the contact layer 108 in the gain region 300a, and the n side electrode 110 is formed on the rear surface of the substrate 101. Finally, cavity facets are formed by cleaving, completing a semiconductor laser as shown in FIG. 13(a).
In the spot size changeable semiconductor laser so fabricated, in the gain part 300, holes supplied from the p side electrode 109 are concentrated in the stripe-shaped ridge structure 113 having a uniform width and then efficiently injected into the active layer 103. In the active layer 103, holes recombine with electrons which are injected from the n side electrode 110, thereby to produce light. This light is guided though the active layer 103 and input to the spot size changing part 200.
In the spot size changing part 200, since the thickness of the active layer 103 gradually decreases in approaching the cavity facet, the amount of light leaking from the active layer 104 into the lower cladding layer 102 and the first and second upper cladding layers 104 and 107 increases in approaching the cavity facet. Therefore, in the spot size changing part 200, the laser light expands in the vertical direction in approaching the cavity facet. As a result, the light emitting spot at the cavity facet broadens in the vertical direction, whereby the width of the far field pattern of the laser light emitted from the facet narrows. Accordingly, in the conventional semiconductor laser, it is possible to make the far field of laser light approximately circular, whereby the coupling efficiency between the laser light and an optical fiber or the like is improved.
Further, in the spot size changing part 200, since laser light traveling through the active layer expands in the vertical direction in approaching closer to the cavity facet, the emission spot of laser light at the cavity facet is increased and the spot size of laser light emitted from the cavity facet is reduced, so that the laser light can be directly applied to an optical fiber or the like. Since it is not necessary to use a lens for reducing the spot size between the optical fiber and the semiconductor laser, the assembly cost is reduced.
Furthermore, since the amount of laser light confined in the active layer 103 is reduced at the cavity facet in the spot size changing part 200, the light density at the cavity facet is lowered, whereby COD caused by light absorption at the cavity facet during high-power output operation can be avoided. As a result, the reliability of the semiconductor laser with the passage of time is improved.
However, in the spot size changeable semiconductor laser shown in FIG. 13(a) wherein the active layer 103 is included in the ridge structure 113 for concentrating the current flow, when the ridge structure 113 is formed by etching and the current blocking layer 102 is re-grown on both sides of the ridge structure 113, side surfaces of the active layer 103 that become re-growth interfaces are exposed to air and oxidized. If the semiconductor laser has such oxidized portions of the active layer 103 to which a current is injected, the energy band gap becomes narrower in the oxidized portions than in the other portions of the active layer, so that some of light generated by current injection into the active layer 103 during the laser operation is absorbed in the oxidized portions, locally increasing the temperature of the laser. If the temperature rises sufficiently, the semiconductor layers contacting both sides of the active layer 103 may begin to melt, resulting in destruction of the internal structure of the semiconductor laser and rapid deterioration of the laser characteristics. As a result, the reliability with the passage of time is degraded.
In the semiconductor laser shown in FIG. 13(a), especially when a mixed crystal semiconductor material containing Al and Ga is used as a material of the active layer to shorten the wavelength of emitted light, oxidation very likely occurs at the side surfaces of the active layer that become re-growth interfaces because Al is an easily oxidizable material. So, it is very difficult to obtain sufficient reliability with the passage of time.